When Jan Koum came to the US from Ukraine as a teenager, his family struggled to stay above the poverty line and relied on food stamps to get by.

Despite his tough upbringing, Koum worked hard at school, and in 2009, he co-founded real-time messaging service WhatsApp. The app was so successful that Facebook bought it for a massive $19.6 billion (£15.4bn) in 2014. Today, Koum is worth $16.1 billion (£12.6bn).